Effective imaging of the retina of an eye depends on the type of retinal tissue that is to be imaged, as well as the optical response of that tissue to the input light beam. In particular, for two specific tissues of the retina, namely the Retina Pigment Epithelium (RPE) and the Lamina Cribrosa (LC), it happens there are two different optical phenomena that generate the particular tissue's response. One is known as Two Photon Excited Fluorescence (TPEF). This phenomenon is efficacious for imaging the RPE of the retina. The other phenomenon is Second Harmonic Generation (SHG), which is efficacious for imaging the LC. An ability to image these tissues (i.e. RPE or LC) depends on how these phenomena are exploited.
Anatomically, RPE tissue in the retina includes the protein, lipofuscin. In the context of the present invention, it is known that lipofuscin is susceptible to TPEF. Specifically, it can be demonstrated that when an input beam of red light (e.g. λi=780 nm) is incident on lipofuscin in the RPE, a resultant return beam of fluorescent green light (e.g. λr1=530 nm) is generated. On the other hand, when this same input beam of red light (λi) is incident on the LC there is a much different response. Specifically, as a result of SHG, a return beam of blue light (e.g. % λr2=390 nm) is generated. (Note: λi≠λr1≠λr2). Nevertheless, both of the return beams (λr1 and λr2) are useable for effectively imaging the respective tissues.
During an imaging procedure, it happens that the anterior components of the eye (i.e. the cornea and the lens) will introduce optical aberrations into the input light beam. Also, the retina will introduce optical and phase aberrations. These aberrations, both optical and phase aberrations, are measurable. Furthermore, using adaptive optics with a wavefront sensor, the input light can be altered to effectively compensate for any optical aberrations that may be present. Further, phase aberrations that are introduced by curvature of the retina can be compensated for by pre-programming input to a computer that controls the adaptive optics.
In light of the above, it is an object of the present invention to provide a system and method that is capable of alternatively imaging the RPE or the LC tissues in a retina of an eye. Another object of the present invention is to provide a system and method that is capable of selectively exploiting the TPEF or SHG phenomenon to image different tissue in the retina of an eye. Yet another object of the present invention is to provide a system and method for imaging selective tissue in the retina of an eye that is easy to implement, is simple to use and is comparatively cost effective.